1. Field of the Invention
This invention relates to controlling the functions of a longwall miner in response to measurement of the coal panel face alignment by electromagnetic trilateration.
2. Summary of the Prior Art
Proper maintenance of face alignment to the axis of the panel is important in maximizing productivity in longwall coal mining. Misalignment can result in kinking the face conveyor into angularly disposed sections making the face conveyor difficult to operate. Further, misalignment to the panel axis will require taper cuts resulting in significant loss of productivity.
In the operating sequence of a longwall miner, the operating face is sequentially advanced through the coal seam. Each section of face conveyor is connected to a powered roof support by a double-acting hydraulic cylinder or ram. After the coal in front of a given face conveyor section is mined, its corresponding roof support is lowered and advanced and then hydraulically "set" against the roof, with the face conveyor section then being advanced. U.S. Pat. No. 4,228,508 illustrates such a longwall mining system.
As the face advances, it frequently becomes misaligned, as different face conveyor sections advance by different amounts. This is caused by a wide variety of intervening circumstances, such as operator error, buildup of floor debris, uneven floor or roof, and deteriorating performance of miner hydraulic components. Misalignment is detrimental to operation of the face conveyor and has been known to result in broken conveyor chains and damaged drive components. Misalignment must be corrected by taper cutting, carefully positioning the face conveyor section by section, so that the taper cut produces a straight face.
Automatic control of face alignment has been the object of much research in Great Britain and France, and several systems have been operated underground. However, each system has had serious difficulty in making consistently accurate measurement of face alignment. There are two general approaches to making this measurement, and each has inherent weaknesses in the operating systems.
The first approach measures the relative position of each shield at a given point in the mining cycle. Some use instrumented advance rams; some measure transit time of reflected light or sound between the passing shearer and targets on the supports or vice versa. In all its forms, this approach requires the installation, calibration, and maintenance of a large number of instruments, a difficult task at best on an operating longwall. In addition, the energy-reflecting methods suffer from the dust and suspended water always present in the ambient air in a longwall face.
The second approach measures the orientation of the shearer as it traverses the face. Since the shearer travels on the face conveyor, its trajectory is a virtual trace of the face line. Some methods have attempted to measure absolute orientation, using a magnetometer; others have measured relative orientation, using a gyroscope or an optical "angle cart" which referenced to the previous pan section. The primary difficulty with all these approaches has been the maintenance of the instruments, most of which have moving parts, mounted on the shearer, where they experience high vibration and severe shock. Even when instruments have survived, it has been difficult to maintain calibration.
Besides the drawbacks which are unique to each approach, there is another problem common to both: in every case, only the relative alignment of the face conveyor is measured, and some secondary method must be used to measure the absolute position of the face in geodetic or mine coordinate systems. To date, the most successful method of doing this has been the "lost-cord" method, in which a steel cable is anchored in the gob and its play-out is measured as the face advances. The method has limited accuracy, and requires careful maintenance. Broken cables, which occur regularly, must be replaced immediately.